In a downlink (DL) of a 3rd generation partnership project long term evolution (3GPP LTE) system and a downlink (DL) and an uplink (UL) of IEEE 802.16 system, an orthogonal frequency division multiple access (OFDMA) scheme is currently used as a multiple access scheme. Also, the 3GPP LTE system has decided to use a single carrier-frequency division multiple access (SC-FDMA) scheme as a multiple access scheme of an uplink (UL). Herein, a system which uses SC-FDMA scheme will be referred to as SC-FDMA system, and a system which uses OFDMA scheme will be referred to as OFDMA system.
Hereinafter, among the multiple access scheme which are generally used, an OFDMA scheme and an SC-FDMA scheme will be described in brief.
The OFDMA scheme means that differently divided subcarrier sets (i.e., subchannels) are allocated to several users. The OFDMA scheme has the following features.
(1) A base station can allocate radio resources to a mobile station by two-dimensionally dividing the radio resources on time and frequency regions using the OFDMA scheme. Accordingly, the respective users (or mobile stations) within the same cell which supports the OFDMA scheme can use different subcarrier sets. In this way, the base station can flexibly allocate the radio resources to the mobile station.
(2) The base station can reduce inter symbol interference (ISI) and inter channel interference (ICI) by allocating different subcarriers to the respective users. For example, if the base station can maintain an influence of frequency and timing offset between the respective users at a sufficiently low level, the base station and the mobile station can little be affected by inter cell interference.
(3) In the OFDMA system, the subcarriers are allocated onto a frequency region in accordance with a transmission speed required by each user, whereby channel capacity can be optimized. Namely, the base station can vary the number of subcarriers, which are allocated in accordance with a transmission speed required by each user, by dynamically allocating the subchannels in accordance with a request of each user.
(4) In the OFDMA scheme, since orthogonality between respective symbols within the same cell is sufficiently ensured, there is no limitation in power control of the base station.
However, in the OFDMA system, OFDM symbols on a time region include a plurality of subcarriers which are independently modulated, and if the respective symbols are synchronously added to each other, the total maximum power is more increased than the average power as much as a multiple of the number of subcarriers. Also, in the general OFDMA system, since input data are processed on a frequency region, it is disadvantageous in that a peak to average power ratio (PAPR) increases if the frequency region is transformed to the time region by IFFT block.
The PAPR is one of main factors to be considered during reverse transmission. If the PAPR increases, cell coverage is reduced. Also, the PAPR is directly associated with power amplifier cost of the mobile station in the uplink. Accordingly, if the PAPR increases, since signal power required by the mobile station increases, it is necessary to first reduce the PAPR during reverse transmission.
The OFDMA scheme has a disadvantage in that the PAPR is generally high. Accordingly, in the uplink of the 3GPP LTE system, the SC-FDMA scheme is used as the multiple access scheme. The SC-FDMA scheme (or DFT-spreading OFDMA) is a transmission scheme that reduces signal variation. A transmitter can add a discrete fourier transform (DFT) module to apply the SC-FDMA scheme so as to relatively reduce overlap of synchronous signals in comparison with the OFDMA scheme, thereby preventing the PAPR from increasing.
Finally, as the DFT module is added to the SC-FDMA system, it is possible to obtain a single carrier feature while maintaining orthogonality. Accordingly, when the same power amplifier is used, the SC-FDMA scheme can use more power for signal transmission. As a result, the SC-FDMA scheme has broad cell coverage.
FIG. 1 is a diagram illustrating a procedure of transmitting and receiving a general signal of SC-FDMA system.
Referring to FIG. 1, a data transmission procedure of the SC-FDMA system will be described as follows. A data coding module 101 of a transmitter codes data to be transmitted. The transmitter inputs the coded data to a DFT module 102 and allocates the data to a frequency region through a sub-carrier mapping module 103. The transmitter again transforms the allocated data to the frequency region to a time region signal through an inverse fast fourier transform (IFFT) module 104, inserts cyclic prefix to the data through a CP module and then transmits the data to a receiver (105).
The data transmitted from the transmitter are transmitted to the receiver through a wireless interface. The receiver deletes the CP from the CP module 106 and transforms a symbol received through an FFT module 107 to a frequency region signal. Afterwards, the receiver can perform inverse fourier transform through an inverse discrete fourier transform module 109 after performing de-mapping through a subcarrier de-mapping (or mapping release) module 108. Also, the receiver can interpret the received data through an inverse coding module 110.
The main feature of the SC-FDMA scheme is that the SC-FDMA scheme has a single carrier effect. Referring to FIG. 1, the transmitter (Tx) spreads a transmission signal through DFT and performs localized mapping for a frequency band in a part where the transmission signal is generated or performs equal spaced mapping with a constant frequency interval so as to obtain a single carrier effect. Accordingly, the transmission signal generated using the SC-FDMA system has a small PAPR.
FIG. 2 is a diagram illustrating a method for mapping subcarrier to obtain a single carrier effect in SC-FDMA scheme.
In the SC-FDMA system, a method for allocating resources can be classified in accordance with a method for transferring the output of the DFT module to the IFFT module. Namely, FIG. 2(a) illustrates a method for localized-mapping radio resources in SC-FDMA system, and FIG. 2(b) illustrates a method for distributed-mapping radio resources in SC-FDMA system.
Referring to FIG. 2(a), the localized mapping is that the output of the DFT module is localized in a specific part of each user when being input to the IFFT module. Accordingly, a subcarrier interval of the output of the DFT module becomes identical with a subcarrier interval of the IFFT module.
Referring to FIG. 2(b), the distributed mapping is that the interval between the respective subcarriers is equally allocated for all frequency bands when the output of the DFT module is input to the IFFT module. If channel change is great on a frequency axis (for example, when delay spread of channel is great), the base station allocates resources on the frequency axis to mobile station in a distributed-mapping mode to obtain frequency diversity in the frequency region. Accordingly, in the SC-FDMA system, the output of the DFT is equally distributed in the frequency region to lower the PAPR.
Referring to FIG. 2, in the SC-FDMA scheme, localized mapping should be performed for subcarriers to obtain a single carrier effect, or distributed mapping should be performed for subcarriers to have a constant interval on the frequency axis. Accordingly, although the SC-FDMA system has a low PAPR during subcarrier mapping, it has a disadvantage in that flexibility is reduced in allocating subcarriers.
The aforementioned multiple access schemes have advantages and disadvantages together. Accordingly, a multiple access scheme that can eliminate the disadvantages and use the advantages is required.